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Bibliography.—Evaporators are used in such great numbers in all branches of the chemical industry that one would expect a considerable literature on this subject, giving data and instructions regarding design, installation and operation. As a matter of fact there is very little to be found in the form of textbooks. There are really only two books: F. Hausbrand, "Evaporating, Condensing and Cooling," an excellent theoretical treatise with some practical information; and Foster, "Evaporation/7 which is merely a collection of older treatises on thermodynamics, and fundamental evaporator patents. Besides, Prof. E. W. Kerr has tabulated and analyzed the results of some well-conducted tests with experimental evaporators in two pamphlets published by the Louisiana State University at Baton Rouge, La., Bulletins No. 138 and 149, and also with several types of sugar evaporators in a paper presented at the meeting of the American Society of Mechanical Engineers, New Orleans, La., April, 1916.
A series of articles, " Studies in Evaporator Design," by W. L. Badger, has also been published in 1920 and 1921 in Chemical and Metallurgical Engineering. Badger gives experimental data with special reference to the horizontal-tube evaporator.
History.—The art of evaporation by direct heat is naturally a very old one, and was used for the manufacture of drugs and pharmaceutical products by the Egyptians as described in the " Papyrus Ebers," 1500 B.C. Sugar and salt were recovered by the same process in the sixth century. The application of direct heat requires only a simple apparatus, but the fuel economy is low, and frequently the liquors are injured by the high boiling temperature. With the development of the cane- and beet-sugar industries, large quantities of water had to be evaporated, preferably at low temperature, and it was found that direct-heated evaporators were entirely inadequate for this work. In the early part of the nineteenth century, Howard and Roth designed and operated the first vacuum pans with condenser, and in 1829 Pecqueur constructed the first multiple-effect evaporator, using as heating surface hemispherical copper bottoms which were placed on top of each other. In 1845, N. Rillieux built the first multiple-effect with horizontal tubes; a few years later Robert constructed the first vertical-tube apparatus. The construction of all modern evaporators is based on these two fundamental types of machines as will be shown in a later chapter. Between 1880 and 1890, a new development took place which led to the introduction of the so-called film evaporators of Claassen, Yaryan, Lillie, Kestner, and others.
Theory of Evaporation.—Evaporation is the art of removing a solvent by application of heat and concentrating the solution until the dissolved substance separates or reaches the desired density. For instance, brine is concentrated
1 Mantius Engineering Co., 15 East 40th St., New York City.